Legionella were first discovered in 1976 following an outbreak of 182 cases of pneumonic illness (termed Legionnaires' disease) occurred at a state convention of the American Legion in Philadelphia, Pa., which resulted in 29 deaths. What is now known as L. pneumophila serotype 1 was originally isolated by standard techniques for the isolation of rickettsiae and subsequently shown to be the etiological agent for Legion-naires' disease, a form of atypical pneumonia with other non-respiratory complications. Since that first outbreak, Legionellae have been implicated in Pontiac fever and Pittsburgh pneumonia. Pontiac fever is a non-pneumonic febrile self-limiting illness caused by L. pneumophila. Pittsburgh pneumonia is a pulmonary legionellosis caused by L. micdadei.
Legionellae are gram negative, aerobic, facultative intracellular, parasitic bacteria found to be practically ubiquitous in fresh water supplies (including evaporative condensers, cooling towers, and potable water). It is believed that Legionellae cause disease when contaminated water is inhaled, often leading to epidemic or clustered outbreaks. In addition to community-acquired cases, legionellae may be a major cause of nosocomial infections. Along with hospitalization, host risk factors include smoking, advanced age, chronic lung disease, and immunosuppression.
The family Legionellaceae contains the single genus Legionella which includes some 29 species, 21 serogroups and 5 tentatively named species. According to DNA homology studies, L. micdadei is the most distant relative of L. pneumophila, and there is some who argue that L. micdadei should be properly classified as a member of the genus Tatlockia.
L. pneumophila, the primary cause of Legionnaire's disease, is the most common human isolate. The presence of legionellae in human clinical samples always provides clinically relevant information, as the bacteria are not considered normal human microflora.
Legionellae are slow growing organisms which are difficult to culture. Thus, isolation of L. pneumophila by laboratories can be difficult and time-consuming. Given the serious nature of the diseases, and the need to prescribe correct antibiotics, it is highly desirable for a physician to make a rapid and accurate diagnosis of the presence of these organisms. Current methods of detection of Legionellae include (a) culture; (b) direct fluorescence antibody (DFA); (c) nucleic acid probes for culture confirmation; and (d) serology (IFA). Serology is currently the most sensitive and specific test method. It is limited however, in that antibodies may persist in an individual's serum for years after infection.
Certain probes which are based on Legionella rRNAs are disclosed in WO 88/03957 "Nucleic Acid Probes for Detection and/or Quantification of Non-Viral Organisms", published Jun. 2, 1988, Applicant: Gen-Probe, Inventors: Hogan et al. This application however, discloses a mixture of three different probes which can be used to differentiate Legionella bacteria from non-Legionella bacteria. No probes suitable for solitary use are reported. It would be desirable to have probes which, when used singly or in pairs can be used in various diagnostic assays involving Legionella.